PLAnetary Transits and Oscillations of stars (PLATO) is a space observatory under development by the European Space Agency for launch in 2026.[2] The mission goals are to search for planetary transits across up to one million stars, and to discover and characterize rockyextrasolar planets around yellow dwarf stars (like our sun), subgiant stars, and red dwarf stars. The emphasis of the mission is on earth-like planets in the habitable zone around sun-like stars where water can exist in liquid state.[3]
It is the third medium-class mission in ESA's Cosmic Vision programme and named after the influential Greek philosopher Plato, the founding figure of Western philosophy, science and mathematics. A secondary objective of the mission is to study stellar oscillations or seismic activity in stars to measure stellar masses and evolution and enabling the precise characterization of the planet host star, including its age.[4]

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PLATO was first proposed in 2007 to the European Space Agency (ESA) by a team of scientists in response to the call for ESA's Cosmic Vision 2015–2025 programme.[5] The assessment phase was completed during 2009, and in May 2010 it entered the Definition Phase. Following a call for missions in July 2010, ESA selected in February 2011 four candidates for a medium-class mission (M3 mission) for a launch opportunity in 2024.[5][6] PLATO was announced on 19 February 2014 as the selected M3 class science mission for implementation as part of its Cosmic Vision Programme. Other competing concepts that were studied included the four candidate missions EChO, LOFT, MarcoPolo-R and STE-QUEST.[7]

In January 2015 ESA selected Thales Alenia Space,[8]Airbus DS, and OHB System AG to conduct three parallel phase B1 studies to define the system and subsystem aspects of PLATO, which were completed in 2016. On 20 June 2017, ESA has adopted PLATO[9] in the Science Programme, which means that the mission can move from a blueprint into construction. In the coming months industry will be asked to make bids to supply the spacecraft platform.

PLATO is an acronym, but also the name of a philosopher in Classical Greece; Plato (428–348 BC) was looking for a physical law accounting for the orbit of planets (errant stars) and able to satisfy the philosopher's needs for "uniformity" and "regularity".[5]

The goal is to find planets like Earth, not just in terms of their size but in their potential for habitability.[3] By using 26 separate small telescopes and cameras, PLATO will search for planets orbiting from 300,000 to one million stars.[7] The main objective of PLATO is to elucidate the conditions for planet formation and the emergence of life. To achieve this objective, the mission has these goals:

Discover and characterize a large number of close-by exoplanetary systems, with a precision in the determination of the planet radius up to 3%, of stellar age up to 10%, and of the planet mass up to 10% (the latter in combination with on-ground radial velocity measurements)

PLATO will differ from the COROT and Kepler space telescopes in that it will study relatively bright stars (between magnitudes 4 and 11), enabling a more accurate determination of planetary parameters, and making it easier to confirm planets and measure their masses using follow-up radial velocity measurements on ground-based telescopes. Its dwell time will be longer than that of the TESS NASA mission, making it sensitive to longer-period planets.

The PLATO payload is based on a multi-telescope approach, involving a set of 24 "normal cameras" working at a readout cadence of 25 seconds and monitoring stars fainter than apparent magnitude 8, plus two "fast cameras" working at a cadence of 2.5 seconds, and observing stars between magnitude 4 to 8.[10] The cameras are refracting telescopes using six lenses; each camera has an 1,100 deg2 field and a 120 mm lens diameter. Each camera is equipped with its own CCDstaring array, consisting of four CCDs of 4510 x 4510 pixels.

The 24 "normal cameras" will be arranged in four groups of six cameras with their lines of sight offset by a 9.2° angle from the +ZPLM axis. This particular configuration allows surveying a total field of about 2,250 deg2 per pointing. The satellite will rotate around the mean line of sight once a year, delivering a continuous survey of the same region of the sky.